git_utils.py: allow checkouts of local repositories at any commithash along master branch

Needed so that we can create new GM/SKP baselines on a shared
rebaseline_server instance, *efficiently*.  (Create checkouts pinned to certain
commithashes as copies of the local Skia checkout, rather than checking out
copies of the main Skia repo across the internet.)

BUG=skia:1918
NOTREECHECKS=true
R=borenet@google.com

Review URL: https://codereview.chromium.org/484143002